The present invention relates to determining visually noticeable differences between two images. The invention has particular application to the encoding and later decoding of an image in such fields as television signal generation, compression and transmission. When encoding an original image for television transmission and display at a receiver, it is important to know how well the system for transmitting and receiving will perform in reproducing the original image. The performance of the system is related to the visually noticeable differences between the original image and the displayed image.
A number of models have already been proposed to predict whether a human observer will be able to discriminate between two images. One such visual discrimination model is known as the Sarnoff visual discrimination model. A visual discrimination model may take, as input, a pair of images and provide an output which is a map showing the probability, as a function of position on the images, that an observer would be able to detect differences between the images. The map is referred to as a JND map showing Just Noticeable Differences. This JND map can itself be presented as an image, with higher grey levels corresponding to higher probabilities of discrimination.
In an encoding algorithm to encode picture information using quantisation methods, the image fidelity should be directly proportional to the quantisation sampling density. This assumption may be erroneous with the result that encoding bits are squandered where they have little impact on the fidelity of reproduction of an image. A better method of controlling quantisation parameters is to base control on a JND map to obtain either more uniform image fidelity for a fixed bit rate of encoding or better bit rates for a desired level of fidelity.
It is an object of the present invention to improve the identification of noticeable differences between two images.
According to the present invention there is now provided a method of identifying visually noticeable differences between a first input image and a second input image, the method comprising the steps of: generating contrast images from each input image; forming a contrast difference image from the contrast images; calculating measures of the mean absolute difference in luminance between blocks of pixels of the first and second input image; selecting pixels from the said contrast difference image which exceed a first threshold so as to form a first threshold output signal; selecting measures of the said mean absolute difference which exceed a second threshold so as to form a second threshold output signal; and, producing a detection signal indicating when both the threshold signals are present.
Further according to the present invention, there is provided apparatus for identifying visually noticeable differences between a first input image and a second input image, the apparatus comprising: a generator to generate contrast images from each input image; a difference forming circuit to form a contrast difference image from the contrast images; a calculator to calculate measures of the mean absolute difference in luminance between blocks of pixels of the first and second input image; a selector to select pixels from the said contrast difference image which exceed a first threshold, so as to form a first threshold output signal, and to select measures of the said mean absolute difference which exceed a second threshold, so as to form a second threshold output signal; and, a detector to produce a detection signal indicating when both the threshold signals are present.
The detection signal is indicative of visually noticeable differences between the first input image and the second input image. Where the second input image is a compressed form of the first input image, the detection signal indicates where noticeable differences occur between them as a result of the compression. The degree of compression can thus be increased in those areas of each image where visually noticeable differences do not appear. Reference is made to our co-pending patent application No. GB 9803580.1 entitled xe2x80x9cDetermining Visually Noticeable Differences Between Two Imagesxe2x80x9d.
The present invention represents an improvement in the invention of the co-pending patent application.